This invention relates to a high-frequency thawing apparatus for supplying a high-frequency power to a pair of opposing electrodes to thaw an article such as a frozen food carried in between the electrodes by dielectric heating.
Japanese Examined Patent Publication Nos. 51-15100 and 55-46152, for example, propose a high-frequency thawing apparatus in which a group of frozen articles to be thawed (hereinafter, merely referred to as "article") are serially carried on a conveyor and introduced into an electric field generated by application of a high frequency to a pair of opposing electrodes while passing therebetween, thereby being thawed one after another.
The conventional thawing apparatuses has such an arrangement that a signal of a high frequency from a high frequency generating circuit is power-amplified by a transformer and the amplified voltage is supplied to one of the pair of electrodes (namely, the high-voltage electrode) for high-voltage application to generate a high frequency electric field between the high-voltage electrode and the other one of the electrodes which is connected to the ground (namely, ground electrode). The article interposed between the high-voltage electrode and the ground electrode is heated by dielectric loss.
Generally, the dielectric constant of the article rises as the thawing is carried on. Namely, the dielectric heating causes an impedance change on time-basis. Accordingly, it is necessary to vary a high-frequency power to be supplied to the article as the thawed state of the article, which is a load in the high-frequency power generating circuit, varies (namely, a matching control of the high-frequency power is necessary). Particularly, since the dielectric constant greatly differs between ice and water, the following problem occurs in the case where the article is expected to be heated up to a temperature around 0.degree. C. after thawing. The amount of water that has been contained in the article in a super-cooled state changes as the article is heated and, accordingly, the above matching control of the high-frequency power supply in the high-frequency thawing apparatus is extremely difficult. It has been practically impossible to uniformly heat a plurality of frozen articles while serially conveyed by using a pair of electrodes or a single power generating device.
In the case where a thick block of article is thawed by heat with use of the thus constructed conventional thawing apparatus, a portion of the article adjoining the high-voltage electrode is apt to be more heated than a portion opposite and away from the high-voltage electrode. Thus, heated state differs portion by portion of the article, thereby making it difficult to uniformly thaw the article.
Further, in the case of transporting the article on the conveyor, it is required to finalize thawing of the article by dielectric heating before the instant article completely passes the space in between the pair of electrodes. This requires to set the high-frequency power at a relatively high level. Consequently, there is a possibility that conspicuous is a so-called "edge effect" due to the electric field generation in which an edge of the article is apt to be more heated than the other part of the article, thereby resulting in a non-uniform thawed state between inner part and outer part of the article.